Long Term Evolution (LTE), which is a succeeding communication system to W-CDMA (Wideband Code Division Multiple Access), HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), has been considered in the W-CDMA standards body (3GPP: Third Generation Partnership Project) and its specifications are now being developed. As a wireless access system in LTE, there are specified the OFDMA (Orthogonal Frequency Division Multiplexing Access) system for the downlink and the SC-FDMA system (Single-Carrier Frequency Division Multiple Access) for the uplink (for example, see Non-Patent Literature 1).
The OFDMA system is a multicarrier transmission system in which a frequency band is divided into plural narrower frequency bands (subcarriers) and data is transmitted on each subcarrier. The subcarriers are arranged orthogonal onto the frequency axis and close to each other and thereby, high-speed transmission is realized and the frequency usage efficiency is expected to be increased.
The SC-FDMA system is a single carrier transmission system in which a frequency band is divided and assigned to terminals and the terminals use different frequency bands in transmission. This system is preferable in terms of lower power consumption of the terminals, wider coverage and the like as interference between the terminals can be reduced simply and effectively and fluctuations in transmission power can be reduced.
In the LTE system, one or more resource blocks are assigned to a mobile station for communications on both of the uplink and downlink. The base station apparatus determines to which mobile station out of plural mobile stations and which resource block to assign for each sub-frame (1 ms in LTE) (this process is called frequency scheduling). In the downlink, the base station apparatus transmits a shared channel to the scheduled mobile station in one or more resource blocks. In the uplink, the selected mobile station transmits a shared channel to the base station apparatus in one or more resource blocks. Here, the shared channel is a PUSCH (Physical Uplink Shared Channel) in the uplink and a PDSCH (Physical Downlink Shared Channel) in the downlink.
Here, there has been proposed a MIMO antenna system, as wireless communication technique, in which data transmission and reception is performed with plural antennas to improve throughput and frequency usage efficiency (for example, see Non-Patent Literature 2). In the LTE system, there are two downlink MIMO modes, that is, space-division multiplexing transmission mode (SU-MIMO: Single User MIMO) and transmission diversity transmission mode. In the space-division multiplexing transmission mode, plural stream signals are spatially multiplexed and transmitted at the same frequency and time, and this is effective in increase in peak data rate. In the transmission diversity transmission mode, a space-frequency (time) encoded stream signal is transmitted from plural antennas. This is effective in improvement of reception quality of cell edge users due to transmission antenna diversity effects.
In the downlink MIMO transmission, in order to increase signal power at the mobile station receiver and improve the throughput performance, when a transmission signal is transmitted via each of plural antennas in the base station transmitter, the phase and/or amplitude of the transmission signal is controlled with respect to each antenna basis (hereinafter, this is referred to as “precoding”).
In the LTE system, at a mobile station receiver, a reception signal from each transmission antenna is used to measure a channel variance and this channel variance is used as a basis to select such a PMI (Precoding Matrix Indicator) to set at an antenna of a base station transmitter that the throughput or reception SINR (Signal to Interference plus Noise Power Ratio) becomes maximum. And this selected PMI is fed back to the base station transmitter. The base station transmitter performs precoding based on the PMI given from the mobile station receiver and then, transmits signals from antennas.
In the above-mentioned MIMO transmission, the channel variance is measured as to all frequency bands of the system bandwidth at the mobile station receiver and the average of reception quality of all frequency bands is obtained to select one PMI, or the system bandwidth is divided into plural sub-bands and an optimal PMI is selected for each of the sub-bands. The selected PMI is fed back to the base station transmitter on the uplink. In order to improve the throughput performance, it is necessary to increase the number of divided bands of the system bandwidth so as to increase the number of PMIs to feed back (feedback information amount).